One known example of a strain wave gearing is a flat strain wave gearing. In a flat strain wave gearing, a cylindrical flexible externally toothed gear capable of meshing with two rigid internally toothed gears having different numbers of teeth is disposed on the inner sides of the rigid internally toothed gears. A wave generator is disposed on the inner side of the flexible externally toothed gear. The flexible externally toothed gear is made to flex into an ellipsoidal shape by the wave generator, and is meshed with both rigid internally toothed gears.
A flat strain wave gearing is in some cases used as a hollow strain wave gearing having a hollow part passing through the center in the along axial direction of the device. Hollow wave generators having rigid plug formed with a through-hole in the center are used as such a hollow strain wave gearing. To increase the inside diameter dimension of the hollow part without increasing the outside diameter dimension of the device, a hollow through-hole having a large inside diameter must be formed in the rigid plug. To achieve this, a roller bearing, particularly a needle roller bearing, of less radial thickness than a ball bearing is preferably used in the wave generator. Patent Document 1 discloses a flat strain wave gearing that has a wave generator using a roller bearing.